Prediction of Residual Stress Evolution for End-To-End Process Chain of Laser Powder Bed Fusion Process and Determination of Fatigue S-N Curves

Abstract

This paper presents prediction of residual stress evolution for end-to-end process chain of laser powder bed fusion (L-PBF) for an aero-casing component made of IN718. The end-to-end process chain includes the simulation of the L-PBF build process, removal of support structures, heat treatment cycle for stress relief, and application of surface-hardening processes such as shot peening and laser shock peening. The simulation of the end-to-end process chain was performed using validated process models for IN718. Validation of the L-PBF process model was carried out for the aero-casing component, where predicted and measured distortions were found to be in good agreement. The predicted residual stresses after each process of the chain were used to develop theoretical fatigue S-N curves using the endurance limit approach. This approach requires knowledge for the surface roughness, as well as the ultimate strength and relative density of the material (defect level). Understanding the evolution of stresses in manufacturing process chains as well as prediction of material properties for functional performance is essential to reduce iterations during the process and product development of L-PBF parts. The advantage of this approach is that S-N curves can be determined very rapidly at every location of the aero-casing or any other component without conducting any fatigue tests physically, which saves considerable cost and time. The S-N curves after the L-PBF build process, heat treatment, and surface hardening were determined and discussed. It was concluded that the proposed S-N curve predictive methodology can be employed in design workflows for estimation of high cycle fatigue in L-PBF process chains very early in the design stage. This would enable designers to mitigate fatigue problems by designing parts more proactively for L-PBF process constraints such as surface roughness, ultimate strength, residual stresses, and relative density or defect level in the given material.